a.swift

public struct PullSignal<T> {
  let operation: ((Result<T>) -> Void) -> Void

  public init(operation: @escaping ((Result<T>) -> Void) -> Void) {
    self.operation = operation
  }

  public func start(completion: (Result<T>) -> Void) {
    operation() { event in
      completion(event)
    }
  }

  public func map<U>(f: @escaping (T) -> U) -> PullSignal<U> {
    return PullSignal<U> { completion in
      self.start { event in
        completion(event.map(f: f))
      }
    }
  }
}